In a conventional weaving loom an operator has to find a heddle to which an abnormal warp belongs by pushing a plurality of heddles aside. It usually takes a long time to find it because the operator has to find it by guess of the place where the abnormal warp is or by a clue of waste of yarn coming out of the loom. Therefore finding a heddle to which an abnormal warp belongs is really troublesome, especially in a dobby which has many heddle frames.